Induction heating is based on varying magnetic field, which induces eddy currents inside an object and whereupon the object is heated by the eddy currents. Typically the varying magnetic field is implemented by coupling electromagnets around the object to be heated with an alternating current generator of 50 Hz or higher frequency.
There are several induction heaters in metal industry used for heating metals for shaping, forming and heat treatments. For example in extrusion of aluminium and copper, metal bars weighing hundreds of kilogram are heated into a suitable temperature before extruding usually with induction heating. However, often the objects are heated beforehand e.g. by an oven and only the last heating is provided by the induction.
The changing magnetic field required for induction heating can be achieved not only by adjusting the strength of the magnetic field but also by changing the direction of the magnetic field in relation to the object to be heated. This can be done either by moving the magnetic field or by moving the object to be heated. Both have their disadvantages. If the objects are large, they are difficult to move, like to rotate enough. If, on the other hand, electromagnets are used, their sufficient balancing and delivery of current is difficult to carry out with required rotational speeds.
Accordingly, one potential alternative has proved to be magnets rotated around the object to be heated or alternatively moving the object to be heated in the magnetic field induced for example by the electromagnets. This prior art in the field of the technique has been presented in the patent publication U.S. Pat. No. 4,761,527.
There are however some disadvantages relating to the known prior art, such as the complexity and inefficiency of the devices. The devices typically have electromagnets, whereupon the power supply for the magnets is needed. In addition a motor or the like is needed in order to provide driving force either for moving the magnets around the object to be heated or moving the object in the magnetic field of the electromagnets. In order to deliver the driving force from the motor either the electromagnets and/or the object an additional driving mechanism is needed, which make the device more complex and also ineffective and expensive. Also the moving the heavy and large object (weighing e.g. hundreds of kilogram) in the magnetic field so that the object is really heated by the induction is very difficult and also unsafe, or at least massive safety arrangement is needed around the moving heavy object.
In addition, with the magnet rotated around the object to be heated the object can be heated essentially to a desired temperature. However, in many embodiments it is not sufficient that the object is heated relatively evenly to a specific temperature. For example a metal or aluminium bar to be extruded, billet, has to be heated in longitudinal direction to have a specific changing temperature profile, so that it can be extruded into a homogeneous profile. In other words the head of the billet has to be warmer and suitable for starting the extrusion whereas the tail has to be colder, so that it doesn't heat too much during the extrusion.